Correcting Non-Manifold Edges in Blender 3D Model for 3D Print
One part of the problem is that you have internal faces which you should delete (see the first picture)
The problem at the circles can be solved with merge by distance
(previously called "remove doubles"). You might also try to recalculate normals (Ctrl+N)
The other pictures show that the borders of some faces are not
connected properly and how you can solve this.
When you see faces which are flickering ("face fighting") when you rotate view, you have two faces that are overlapping and sould delete one of them.




Happy Blending! :)